An example of the use of such network materials in the form of a sandwich structure formed from two solid cover sheets and a core formed from a knitted fabric deep-drawn into a well structure and provided with synthetic resin is described in EP-A-158 234.
To produce such deep-drawable sheet materials, DE-A-3 844 458 (HOE 88/F 386) proposes a wrapped yarn composed of a core yarn of low stability and a high-tenacity sheath yarn.
The high stability of this textile material under normal handling and in finishing processes combined with a very good deep-drawability results from the advantageous structure of the material formed from the wrapped yarn. Under normal handling, and for example in the course of finishing processes, any tensile forces are absorbed by the core thread of the wrapped yarn, ensuring a high dimensional stability of the textile material. If, by contrast, considerably elevated tensile forces are exerted on the material in the course of a process of deep-drawing, the core threads of the wrapped yarn break at random places within the areas to be deformed and release a corresponding length of the sheath thread. This mechanism in response to deep-drawing permits an appreciable enlargement in area without destroying the integrity of the area as a whole.
The mechanism described can be further augmented by using core threads having a lower stability than the sheath filaments, i.e. by wrapping the core thread with a yarn which is the actual strength component but which is incorporated in the wrapped yarn in a distinctly greater length. On deforming the sheet material according to the invention, the core thread is destroyed by the mechanical stress, which may be accompanied by an additional thermostress, and/or by the effect of chemicals, and the previous sheath yarn is stretched and then takes over the load-bearing function in the sheet material.
Despite all their advantageous properties these sheet materials have the disadvantage that wrapped yarns are very expensive to manufacture.